1. Field of the Invention
The present invention relates to a sub woofer system wherein a sub woofer, the output sound signal of that has been produced by digital signal processing involving therein a group delay, is connected to an analog audio reproduction system.
2. Description of the Related Art
In a speaker system of an audio reproduction system, in order to obtain the range of plentiful low sounds, it has hitherto been carried out to construct the speaker system by adding a sub woofer to a main speaker unit. Ordinarily, as the low pass filter used when adding the sub woofer, there is used an analog passive filter comprised of a coil, a capacitor, and a speaker, or an analog active filter comprised of an amplifier, a resistor, and a capacitor.
Generally, a voice band passed through the added sub woofer is set so as to cover from 40 to 200 Hz inclusive. Therefore, the voice band that come above that frequency band are eliminated by the low pass filter. As the low pass filter, there are two types, one of them is an analog type in which although no group delay occurs the phase varies with the filter gain, and the other is a digital type in which group delay occurs. Many of the low pass filters that have been generally adopted are the analog-type ones in which no group delay occurs.
However, in the low pass filter of analog type, when using one having a steep filter characteristic, the quality of sounds tend to be degraded. For this reason, it is said that the limitation upon the filter characteristic that can be practically used is to an extent of 24 db/oct.
Also, the followings are known, too. Namely, because of the filter characteristic of the analog type filter, the sound at the crossover frequency and the sounds whose frequencies are in the vicinity thereof within the inhibition band of the sub woofer remain to exist while being subjected to rotation of the phase. These residual sounds make phase interference with the sounds within the pass band of the main speaker. Resultantly, adding the sub woofer causes degradation of the quality of the sound range undertaken by the main speaker.
On the other hand, in the digital type in which group delay occurs, a filter characteristic that is 10 times or more steeper than that of the analog type can be realized at a real time by the use of a DSP (Digital Signal Processor) or a CPU. Namely, by using an FIR type or IIR type digital filter in which there is used the DSP or CPU, it is possible to realize a steep filter without being accompanied by a degradation in the quality of sound that would occur in the analog type low pass filter. The reason for this is as follows. Namely, because the sounds within the inhibition band of the sub woofer are rapidly attenuated, the interference between the sounds within the inhibition band of the sub woofer and the sounds within the pass band of the main speaker occurs only to a small degree. As a result of this, it is possible to prevent degradation in the quality of sound within the range of sound undertaken by the main speaker.
However, in the digital type, at the crossover point between the main speaker and the sub woofer, the sound at the point is output from both the sub woofer and the main speaker substantially in same quantity. Therefore, phase interference occurs between the two due to the group delay in a real time digital signal processing part of the sub woofer. This causes a problem that the quality of sound at this crossover point degrades.
The present invention has been made in view of the above and has an object to provide a sub woofer system which has a construction that enables arbitrarily setting the group delay time length in the digital type real time digital signal processing part to thereby enable improving a phase interference between a main speaker and the sub woofer at a crossover point.
To attain the above object, according to the present invention, there is provided a sub woofer system which is mainly characterized in that the sub woofer system comprises a real time digital signal processing part that includes an A/D converter block, a low pass filter block, a delay block, and a D/A converter block, an analog power amplifier, and a speaker, whereby processing in the digital signal processing part is executed by setting the delay time length in the delay block so that the group delay time length in the digital signal processing part may be equal to an integral multiple of the time length corresponding to one wavelength of a crossover point frequency between the sub woofer system and a main speaker.
The present invention, in the above-described construction, may be arranged to enable automatically setting the group delay time length and thereby enable manually minutely adjusting the set time length thereof. Also, in place of the above-described construction, the invention can be made up into the following construction, too.
Namely, a first example of the other constructions of the sub woofer system according to the invention is a sub woofer system (FIG. 2) having a construction comprising a real time digital signal processing part that includes an A/D converter block, a low pass filter block, a delay block, and a D/A converter block, an analog power amplifier, and a speaker. In the sub woofer system, the delay time length in the delay block is automatically set so that the group delay time length in the digital signal processing system may be equal, among the time lengths represented by integral-multiple values of the time length (e.g. {fraction (1/100)} sec. in case of 100 Hz) corresponding to one wavelength of the crossover point frequency between the sub woofer and the main speaker, to the shortest time length that when the delay time length in the delay block is set to be zero is among the time lengths larger than the group delay time length, whereby the above-mentioned delay time length in the delay block is made manually minutely adjustable.
Also, a second example of the other constructions is a sub woofer system having a construction comprising a real time digital signal processing part that includes an A/D converter block, a low pass filter block, a delay block, and a D/A converter block, an analog power amplifier, and a speaker. In the sub woofer system, the delay time length in the delay block is automatically set so that the group delay time length in the digital signal processing system may be equal, among the time lengths represented by integral-multiple values of the time length (e.g. {fraction (1/100)} sec. in case of 100 Hz) corresponding to one wavelength of the crossover point frequency between the sub woofer and the main speaker, to the shortest time length that when the delay time length in the delay block is set to be zero is among the time lengths larger than the group delay time length; and with respect to the delay time length, there is input as a manual operation, or a system operation, offset value the difference between the distance from a listener to the main speaker and the distance from the listener to the sub woofer, and this difference is calculated in terms of the time length, whereby the calculated result is reflected by the group delay time length, and simultaneously this calculated time length is made manually minutely adjustable. In the present invention, as the factor reflected by the group delay time length there is not only the difference in distance between the two speakers but also, in some cases, the difference between a phase angle of the main speaker itself and that of the sub woofer itself. Therefore, in the present invention, there is also a case where this difference in phase angle is calculated in terms of the time length and is reflected by the group delay time length.
In the sub woofer system of the present invention, the A/D converter block may be replaced with a digital audio interface block (DAI block), whereby it may be arranged that a digital signal transmitted from a reproduction medium such as a CD, a DVD, etc., a digital pre-amplifier, and a digital channel divider, etc. can be directly input to the sub woofer system.
Further, in the sub woofer system of the present invention, to the real time digital signal processing part, there is added an equalizer block for correcting to flatness the peaks and dips of the frequency response and/or phase response that is specific for a speaker unit used in the sub woofer or that is produced with a combination of the sub woofer and a speaker box, whereby, if necessary, the correction has its contents made manually minutely adjustable for producing a higher quality of sounds (FIG. 4).
Additionally, in the present invention, the equalizer block to be added is in some cases used as means for correcting to flatness the descent in a low-sound range and making this correction manually minutely adjustable. It should be noted that in the present invention, the power amplifier for use for the sub woofer can be replaced with an another arbitrary power amplifier. Further, in the present invention, in a sub woofer system having an arbitrary power amplifier and an arbitrary sub woofer in combination, the real time digital signal processing part is inserted to thereby construct the sub woofer system of the invention.